1. FIELD OF THE INVENTION
The present invention relates generally to recording apparatus or recorders of the so-called thermographic or heated stylus type, wherein an electrically heated stylus, also known as a hot pen, marks or produces a trace on a heat-sensitive record medium in accordance with data to be recorded. Specifically, the invention relates to the electrical apparatus and circuitry which are employed to control the energization of the heaters of such heated styli in such recorders.
2. DESCRIPTION OF THE PRIOR ART
Various forms of recorders employing heated styli, and various forms of such styli, have been proposed in the past for various applications. Examples of different forms of such known recorders, heated styli, and/or stylus heater energizing circuits are found in the Rich U.S. Pat. No. 3,002,802, the Bjork U.S. Pat. No. 3,438,056, the Phillips et at U.S. Pat. No. 3,689,937, the Eck et al U.S. Pat. No. 3,699,588, and the Shimotsuma U.S. Pat. No. 3,813,677.
Among the various forms of heated styli which have been employed in the known forms of heated stylus recorders, the heated tip or hot tip type of stylus has proved to be quite attractive and hence quite popular. This is particularly true of the form of hot tip stylus which is shown in the above-noted Eck et al patent. This popularity of the hot tip type of stylus has been due, at least in part, to its low thermal inertia and hence fast response, which give this type of stylus, among other things, the ability to provide intermittent or interrupted records or traces without the need for pen lifters or other mechanical trace-interrupting devices.
The desirable characteristics of the hot tip stylus are due, at least in part, to its marking or writing tip being the heating element or heater of the stylus. In the Eck et al. form of stylus, for example, the stylus consists of a ceramic mandrel having a dome-shaped end or tip which carries a coating or deposit of electrical resistance material, such as a ceramic ink. A pair of flexible electrical conductors connects this coating to a suitable source of electrical energy so that this coating constitutes the stylus heater. The electrical resistance of such a coating will be referred to herein as the stylus heater resistance, or simply as the heater resistance.
The stylus is so mounted and supported in its recorder carriage that the noted coating is maintained in appropriate thermal contact with the associated heat-sensitive record medium as the stylus is moved and positioned along its path of travel across the medium. When it is desired to have the stylus mark the medium, a voltage from the noted source is applied across the flexible conductors and the coating, causing the latter to become sufficiently hot to provide the desired marking and recorded trace.
The operational goal of a heated stylus recorder is that it produce a trace of consistently uniform density or intensity and appearance whenever the stylus heater is energized to place the stylus in the writing condition. This requires that the input power to the heater or the heater power be maintained at the value which produces the desired trace intensity for the prevailing conditions of record medium speed, stylus speed, etc. This, in turn, requires that the heater energizing voltage and current be continuously kept commensurate with the heater resistance as needed to provide the desired value of heater power. For simplifying the present description, the latter will refer to the adjustment of the heater energization or the heater voltage as the adjustment which keeps the heater power at the desired value for different heater resistances.
For a stylus of the above-described type, the heater resistance increases progressively as the stylus wears in use, since stylus wear means wear and erosion of the heater. Consequently, in order to keep the recorded trace intensity consistenly uniform in the presence of such increasing heater resistance, and thus prevent trace degradation, it is necessary to adjust the heater energization correspondingly as the stylus wears.
Additionally, it has been found to be impractical, due to the costs involved, to produce the desirable styli of the above-described coated tip type with a sufficiently consistent heater resistance from unit to unit to assure that such styli can be placed or interchanged in the known recorders without requiring a heater energization adjustment to be made at the appropriate time in order to secure or preserve the uniformity of the produced trace. This variation in heater resistance from stylus to stylus is believed to be due to the nature of the electrical resistance material which is employed, and the manner in which it is used, in the manufacture of the styli.
For all of the known forms of heated stylus recorders and their stylus heater energizing circuits, the above-described heater energization adjustments, to secure and/or preserve uniform trace intensity for different heater resistances, must be made manually. The reason for this is that none of the known recorders automatically adjusts its stylus heater energization to compensate for differences in heater resistance to the end of maintaining a uniform trace intensity notwithstanding such resistance differences.
The above-described requirement for manual heater energization adjustment imposed by the known recorders amounts to a practical problem for several reasons. In the first place, this requirement means that, to prevent trace degradation with stylus use, the known recorders must be continually manually adjusted to maintain the proper heater input power as their styli wear. Additionally, said requirement adds to the cost of producing such a recorder, since it necessitates the manual adjustment of the heater energization in each recorder individually after its stylus is installed. More seriously, said requirement imposes a substantial burden on a user of such a recorder when it becomes necessary to replace the stylus in the field, where the needed manual heater energization adjustment to match the resistance of the new stylus must generally be carried out on a trial-and-error basis.
Since it is impractical to so carry out the above-noted stylus manufacturing process as to keep the heater resistances sufficiently uniform from stylus to stylus to obviate the need for manually adjusting the stylus heater energization in the known recorders to match the particular stylus being used, and since it is clearly disadvantageous to have to make such manual heater energization adjustments, as well as those required to compensate for stylus use and wear, the failure of the known recorders to effect automatic compensation for differences in stylus heater resistance amounts to a series shortcoming of such recorders.
In summary, the known heated stylus recorders, when employing the desirable type of heated stylus described above, are characterized by a shortcoming which results in three significant drawbacks. The first of these is the necessity for manually adjusting each recorder at the time of its production to match the resistance of the particular stylus which is initially incorporated in the recorder. The second of these is the necessity for manually adjusting a recorder in the field when it is necessary to replace its stylus. The third of these is the necessity for manually adjusting the recorder to offset the progressive change in stylus resistance and accompanying degradation of the recorded trace which occurs as the stylus wears. While the first two of these drawbacks could be avoided to some extent by utilizing manufacturing techniques which narrow the resistance spread of the manufactured styli, such techniques make the styli unduly costly.